Vital signs of a person, for example heart rate (HR), respiration rate (RR) or oxygen saturation (i.e. SpO2), serve as indicators of the current state of a subject (i.e. a person or animal) and as powerful predictors of serious medical events. For this reason, vital signs are monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings. Various sensors can thus be used to measure a vital sign information signal from which a corresponding vital sign can be obtained.
One way of measuring vital signs is plethysmography. Plethysmography typically refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heart beat. Photo-plethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise further embedded information attributable to respiration and further physiological phenomena. By evaluating the transmissivity and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters for measuring the heart rate and the oxygen saturation of a subject are attached to the skin of the subject, for instance to a finger tip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red and an infrared LED as light sources and a photodiode for detecting light that has been transmitted through patient tissue. The transmissivity in the red and infrared spectral range is measured by time multiplex. The transmissivity over time gives the red and infrared PPG waveforms.
It is well known that the frequent occurrence of false medical alarms in the hospital, e.g., alarms generated by patient monitoring devices in the intensive care unit (ICU), presents a serious and unresolved problem because it leads to a desensitization of the caregivers against alarms. Furthermore, it is known that the high sensitivity of modern patient monitoring systems leads to alarm noise levels around 80 dB in today's average ICUs, which is comparable to the traffic noise on a main street. However, up to 90% of the registered alarms are medically irrelevant. The technical alarms, which make up about 22% of all alarms, are often due to bad sensor signals due to patient motion. This is especially the case for SpO2-related alarms, which make up 79% of the technical alarms.
It appears therefore beneficial to obtain relevant information on the sensor's motion and to utilize this information to reduce the false alarm probability. This is particularly important in conjunction with “cable-less patient” approaches, where the patient can move around freely in the hospital.